The phenotypes and gene frequencies of two serum protein systems (GC, C3) were studied in 238 consecutive patients with peptic ulcer from all over Greece. 173 patients had duodenal ulcer and 65 had gastric ulcer. Healthy Greeks studied previously in our department served as controls. No significant differences were found between the studied groups and the controls as far as GC was concerned. However, significant differences emerged in C3; the C3*F gene was almost twice more common and the C3F phenotype almost three times more frequent in patients than in controls.